In order to move a shipping container, a spreader which is attached to a crane, will engage the container at four peripheral points on the upper portion of the container. The engagement of the spreader and container is achieved by what is termed a twist lock engagement which is arranged to provide a quick engagement and disengagement arrangement. The engagement of said twist locks between the spreader and container, however, require a degree of precision which may not be readily available subject to environmental conditions.
To assist the crane operator, flipper assemblies are used to contact the container and guide the spreader so as to align the twist lock engagement between the spreader and container.
Where a container has sufficient clearance around it, typically a spreader will approach from above with all flippers down. Flaring of the bottom portion of the flipper envelopes the corners of the container and permitting the spreader to slide down onto the container using the length of the flippers as a guide. Alternatively if the container does not have sufficient clearance, a spreader may approach from the side with two flippers up and two flippers down. In the “up” position the flippers are clear of the spreader and container and do not participate in the guiding action. The spreader is moved horizontally into proximity with the corners of the container, and then lowered as before using the two down flippers as guides.
When the flippers contact the container, a corresponding impact force is applied. To avoid damage to the flippers from such impact force, particularly if a circumstance leads to a particularly high impact force, the flippers are permitted to “back drive”, that is, when a preset torque about the flipper hinge is exceeded corresponding to an unusually high impact force. The back drive capability for a conventional hydraulic flipper is achieved by providing a pressure release within the hydraulic circuit such that on exceeding the pressure, a release operates permitting free rotation of the flipper.
For flippers driven by electric motors, this is more difficult. The corresponding analogy to a pressure release for an electric motor is permitting the gear box to reverse drive when a certain applied force is exceeded. One such measure involves the gear box to back drive by disengaging and operating the motor without a brake and so allow the flipper to move back freely. In this case, the flipper provides no resistance and thus can be back drive freely but is effectively useless as a guide. This lack of a holding torque at the flipper down position is different from that of a hydraulic flipper in that at least a braking pressure can be maintained with a pressure release.
It would, therefore, be preferable to not have the flipper move freely at the down position. Accordingly such systems maintain a brake used at the motor end to brake the motor when the flipper is in the down position as so provide a holding torque.
However, this arrangement can cause substantial damage to the gear box unless safety measures are incorporated. Particularly if the subsequent torque applied to the back drive is high or for a prolonged period.
A further problem with flipper design according to the prior art is the propensity for the motor and gear box to be damaged during operation. With the motor and gear box mounted at the corner of the spreader so as to drive the flipper through the hinged mounting of the flipper to the spreader, this places the motor and gear box proximate to the location of high impact loads either through the flipper or from external contact with other objects.
Further, in order to drive directly through the hinge mounting, it is necessary to locate the motor and gear box at the corner of the spreader so as to be mounted with the flipper. This further reduces the size of the motor and gear box and so limits the rating of either leading to a customised design of the motor and gear box in order to compromise between the contrary design parameters of size and rating.
The prior art shows a motor drive the flipper using a worm gear so as to maintain control and be consistent with the direct drive through the hinged mounting of the flipper. However, whilst applicable to this motor/gear box/flipper arrangement, a worm drive is not useful to provide a back drive capability.